DACUS, (D ash A ttack C anceled U p S mash), is a technique that can be used by a few characters.

The only one I've ever been able to do it with is Falco. Here's how. Run at the opponent, use your dash attack when you're close to them (first frame of attack must his opponent), and immediately afterwards, you tilt the C-Stick up, if that's what Smashes are set to. This is very useful when you need your opponent to get more damage, and can be used as parts of combos, such as...

Grab, Down Throw, Run at opponent, DACUS. (Only attempt at low percentages)

Usually the opponent would use their down aerial when you have knocked them into the air, so you should use your shield after they have been knocked upwards. Right before they land, you should use your Down Smash. I use this all the time as Falco and it sometimes turns the table over, making me in the lead.

Remember, that this is only for Falco. There are others that can do this technique, but not in the same actions. I haven't been able to pull it off with anybody else.